Silicon-based, broadband, waveguide-integrated electro-optical switch

ABSTRACT

An electro-optical switch or router includes a semiconductor oxide substrate and first, second, and third semiconductor waveguides disposed on the semiconductor oxide substrate. The third waveguide includes a transparent conducting oxide layer, an oxide layer, a metal layer, and first and second electrodes coupled to the third waveguide. The electrodes are configured to bias and unbiased the third waveguide to effect optical switching in the electro-optical switch. The oxide layer is disposed between the transparent conducting oxide layer and the metal layer. The switch may further include a semiconductor layer disposed under the transparent conducting oxide layer between the transparent conducting oxide layer and the semiconductor oxide substrate. The first electrode may be coupled to the transparent conducting oxide layer, and the second electrode may be coupled to the metal layer.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/940,999, entitled “A Compact Plasmonic MOS-Based Electro-Optic 2×2Switch” and filed Feb. 18, 2014, the entire contents of whichapplication are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a silicon-based, broadbandwaveguide-integrated electro-optical switch for performing opticalswitching and, more specifically, to an electro-optical switchcomprising a Metal Oxide Semiconductor (MOS)-stack for opticallycoupling and decoupling integrated silicon-based waveguides.

BACKGROUND OF THE INVENTION

The success and ongoing trend of on-chip optic integration anticipate aphotonic road-map leading to compact photonic integrated components andcircuits [1]. An on-chip polarization splitter that is important toachieve polarization independent operation has been proposed withvarious approaches. These approaches overcome the drawbacks of largedevice footprint for the adiabatic mode evolution [2], the disadvantageof clamped bandwidths, and the tighter fabrication tolerances [3] formode coupling based devices.

Passive device architectures rely on the availability of twopolarizations to provide switching functionality. In order to reduce thedevice footprint and switching power, i.e. voltage and capacitance, thelight-matter-interaction must be enhanced. To achieve this goal avariety of techniques are possible, ranging from high-field densitywaveguide modes, such as slots, and introducing optical cavities, toplasmonic approaches [4-15]. It has been previously shown that thestrong electro-optical effects in metal-oxide-semiconductor (MOS)-likedevice designs on a silicon-on-insulator (SOI) low-optical-lossintegration platform are realizable [16].

SUMMARY OF THE INVENTION

In accordance with an exemplary aspect of the present invention, thereis provided an ultra-compact, silicon-based, broadband,waveguide-integrated electro-optic switch for performing opticalswitching, i.e. path routing. The device is based on a MOS design in thetelecomm O, E, S, C, L, U-band wavelengths. Through active tuning of thecarrier concentration of a nanometer-thin indium tin oxide (ITO) layersandwiched inside the MOS structure, an active three-waveguide switchfor transverse magnetic (TM) polarized light is enabled. Classical weakoptical coupling (such as waveguide-to-waveguide) is strongly enhancedby the deep-subwavelength optical mode of the hybridized plasmons, i.e.the plasmonic MOS mode [13-16]. The switching functionality is achievedby the ITO's capability of changing its imaginary part of the refractiveindex by several orders of magnitude, shifting the effective index ofthe optical mode and hence altering the modal overlap betweenneighbouring waveguides [16].

In accordance with another exemplary aspect of the present invention,there is provided an electro-optical switch comprising a semiconductoroxide substrate and first, second, and third semiconductor waveguidesdisposed on the semiconductor oxide substrate. The third waveguidecomprises a transparent conducting oxide layer, a semiconductor oxidelayer, a metal layer, and a pair of electrodes coupled to the thirdwaveguide and configured to bias and unbiased the third waveguide toeffect optical switching in the electro-optical switch.

In accordance with another exemplary aspect of the present invention,there is provided a method for optimizing coupling between the first,second, and third waveguides of the electro-optical switch. The methodincludes steps of performing an eigenmode analysis at a cross-sectionthrough the first, second, and third waveguides, determining a optimizedcoupling length for a cross state of the electro-optical switch,analyzing an effect of changing a width of a first gap between the firstand third waveguides and a width of a second gap between the second andthird waveguides, analyzing an effect of a width of the third waveguideand a height of the semiconductor layer, and calculating an extinctionratio between an output port of the first waveguide and an output portof the second waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustration, there are shown in the drawings certainembodiments of the present invention. In the drawings, like numeralsindicate like elements throughout. It should be understood that theinvention is not limited to the precise arrangements, dimensions, andinstruments shown. In the drawings:

FIG. 1 shows a perspective view of an electro-optic switch comprisingthree optical waveguides, one of the three optical waveguides comprisinga conductive transparent oxide, in accordance with an exemplaryembodiment of the present invention;

FIG. 2A shows a schematic diagram of a view of a cross-section of theswitch of FIG. 1 taken along a plane generally designated as A′ in FIG.1, in accordance with an exemplary embodiment of the present invention;

FIG. 2B shows a schematic diagram of a view of a cross-section of theswitch of FIG. 1 taken along a plane generally designated as B′ in FIG.1, in accordance with an exemplary embodiment of the present invention;

FIG. 2C shows a top plan schematic view of the switch of FIG. 1, inaccordance with an exemplary embodiment of the present invention;

FIG. 3A shows a performance of the switch of FIG. 1 as a function of agap, g, in accordance with an exemplary embodiment of the presentinvention;

FIG. 3B shows a performance of the switch of FIG. 1 as a function of anisland width, W_(island), in accordance with an exemplary embodiment ofthe present invention;

FIG. 3C shows a performance of the switch of FIG. 1 as a function of asilicon core height, h_(island), in accordance with an exemplaryembodiment of the present invention;

FIG. 4A shows a normalized power transmission through the switch of FIG.1 in a cross state as a function of various thicknesses of theconductive transparent oxide layer (specifically an ITO layer in FIG.4A) of the switch of FIG. 1, in accordance with an exemplary embodimentof the present invention;

FIG. 4B shows a normalized power transmission through the switch of FIG.1 in a bar state as a function of various thicknesses of the conductivetransparent oxide layer (specifically an ITO layer in FIG. 4A) of theswitch of FIG. 1, in accordance with an exemplary embodiment of thepresent invention;

FIG. 5A illustrates an electric field profile distribution over theswitch of FIG. 1 in an xy plane through a middle of the conductivetransparent oxide layer (specifically an ITO layer in FIG. 4A) when theswitch of FIG. 1 is in a cross state with a voltage bias, in accordancewith an exemplary embodiment of the present invention;

FIG. 5B illustrates an electric field profile distribution over theswitch of FIG. 1 in an xy plane through a middle of the conductivetransparent oxide layer (specifically an ITO layer in FIG. 4A) when theswitch of FIG. 1 is in a bar state with a voltage bias, in accordancewith an exemplary embodiment of the present invention; and

FIG. 6 is a flow diagram for optimizing compiling performance.

DETAILED DESCRIPTION OF THE INVENTION

Reference to the drawings illustrating various views of exemplaryembodiments of the present invention is now made. In the drawings andthe description of the drawings herein, certain terminology is used forconvenience only and is not to be taken as limiting the embodiments ofthe present invention. Furthermore, in the drawings and the descriptionbelow, like numerals indicate like elements throughout.

Referring now to FIGS. 1, 2A, 2B, and 2C, there are illustrated variousviews of a photonic router or electro-optic switch, generally designatedas 100, in accordance with an exemplary embodiment of the presentinvention. The switch 100 comprises a semiconductor oxide substrate 110and three waveguides, including a first end waveguide 120, a second endwaveguide 130, and a center waveguide 140. Optical light signals travelthrough the waveguides 120, 130, 140 carrying data. The switch 100 is a2×2 switch, whereby the center waveguide 140 switches light signalsbetween the two end waveguides so that the light signals travel withinone of the end waveguides 120, 130 or passes from one of the waveguides120, 130 to the other of the waveguides 120, 130. The substrate 110 is alow dielectric layer that serves as a substrate cladding, such as SiO₂or Al₂O₃. The substrate 110 can be a Silicon “handle” wafer, and theentire starting structure is called SOI=silicon-on-insulator, with the2(3) Si waveguides are the top epitaxial silicon layers. However, othersuitable structures can be used, such as III-V or II-VI semiconductorstructures.

The first end waveguide 120 has a height, h₁, and a width, w₁; thesecond end waveguide 130 has a height, h₂, and a width, w₂; and thecenter waveguide 140 has a height, h₃, and a width, W₃. In an exemplaryembodiment, the end waveguides 120 and 130 are formed from silicon, andthe semiconductor oxide substrate 110 is formed from SiO₂. In a furtherexemplary embodiment, h₁ and h₂ are each 250 nm, and w₁ and w₂ are each450 nm. In one embodiment, h₁ and h₂ are not less than 220 nm, which isthe cut-off at λ=1310 nm, and 250 is the value for light going through.However, other suitable dimensions can be utilized within the spirit andscope of the invention.

As shown, the center waveguide 140 is disposed on the substrate 110between the two end waveguides 120, 130. Each of the waveguides 120,130, 140 is elongated, with a length that is substantially greater thanthe width and height, and each has a respective longitudinal axis thatextends along the length of the respective waveguide. The waveguides120, 130, 140 are arranged to be parallel to one another, with thelongitudinal axis of each waveguide 120, 130, 140 being parallel to thelongitudinal axis of each other waveguide 120, 130, 140. The centerwaveguide 140 is separated from the first end waveguide 120 by a gap 125of width, g₁, and from the send end waveguide 130 by a gap 135 of width,g₂. The center waveguide 140, therefore, is an “island” between the twoend waveguides 120, 130. In an exemplary embodiment, the centerwaveguide 140 is substantially centered between the two end waveguides120, 130. Thus, the gap 125 of width, g₁ is equal to the gap 135 ofwidth, g₂.

As best shown in FIG. 2C, each of the waveguides 120, 130, 140 has twotransverse ends, two longitudinal sides, a top and a bottom. The switch100 is, therefore, a 2×2 switch for switching light input at either ofthe two inputs 121, 131 to either of the two outputs 122, 132. Thetraverse ends include a first transverse end 121, 131 and a secondtransverse end 122, 132 opposite the first end 121, 131, respectively.And the longitudinal sides include a first side 123, 133 opposite asecond side 124, 134, respectively. Thus, the first waveguide 120 has afirst end footling an input 121 and a second end forming an output 122for TM polarized light, and the second waveguide 130 has a first endforming an input 131 and a second end forming an output 132 for TMpolarized light. The center waveguide 140 has a first transverse end 141and a second transverse end 142 opposite the first end, and a firstlongitudinal side 143 opposite a second longitudinal side 144.

Each of the waveguides 120, 130, 140 has a rectangular shape and isarranged parallel to one another. Accordingly, the sides 123, 124, 133,134, 143, 144 each lie in a respective plane, and the planes areparallel to each other. In addition, one side 124 of the first waveguide120 faces the first side 143 of the center waveguide 140, and one side133 of the second waveguide 130 faces the second side 144 of the centerwaveguide 140. And as best shown in FIG. 2C, the center waveguide 140 issubstantially shorter in length than the length of the two endwaveguides 120, 130. The center waveguide 140 is arranged to at leastpartially overlap with each of the two end waveguides 120, 130, and canbe centered with respect to the lengths of the one or both of the endwaveguides 120, 130. Though the end waveguides 120, 130 are shown tohave equal lengths and are aligned with one another in the embodiment ofFIG. 2C, they can have different lengths or can be offset from oneanother. The end waveguides 120, 130 can have the same height and widthas shown. The dimensions of the center waveguide 140 will be discussedin more detail below, but the center waveguide 140 can have any suitablelength, height and width, including that the height is greater (FIGS. 1,2A, 2B) and the width is smaller (FIG. 2C) than that of the two endwaveguides 120, 130. The height of the center waveguide (145) improvesthe switch performance; that is, the optical mode profile overlap (i.e.the electric field density) between the two waveguide busses (i.e. 120,130) with the local optical mode profile of the system comprised by theisland region (i.e. 145-148) is better when the physical height of layer145 is lower relative to the height of waveguides 120, 130. This socalled ‘detuning’ increases the mode profile overlap and hencestrengthens the interaction of the global optical system (i.e., opticalsupermodes).

Referring to FIGS. 1, 2A, 2B, the two end waveguides 120, 130 are asingle layer of a single homogeneous material, though any suitablewaveguides can be provided. The center waveguide 140 has a bottom layer145, two middle layers 146, 147, and a top layer 148, stacked one on topof another. The bottom layer 145 can be a high dielectric layer 145 suchas a semiconductor, the first middle layer 146 can be a TransparentConducting Oxide (TCO) layer 146 such as an Indium Tin Oxide (ITO), thesecond middle layer 147 can be an low dielectric layer 147 such as asemiconductor oxide, and the top layer can be a metal. Thus, the metallayer 148 is positioned over the oxide layer 147, which is positionedover the ITO/TCO oxide layer 146, which is positioned over thedielectric layer 145. The layers 145, 146, 147, 148 have the same widthand length as each other, so that each layer covers the entire adjacentlayer. A first electrode 162 is coupled to the semiconductor layer 145or TCO layer, and a second electrode 164 is coupled to the metal layer148. Active switching in the device 100 between the inputs 121, 131 andthe outputs 122, 132 occurs at the center island 140 in the control of abias voltage or the absence of a bias voltage bias applied thereto,specifically via the electrodes 162, 164. In one embodiment, thesemiconductor layer 145 has a height equal to or less than that of thetwo end waveguides h₁ and h₂; the TCO layer 146 thickness is in therange of about 10 nm to 80 nm; the height of the oxide layer 147 isdetermined based on the equation discussed below; and the metal layer148 has a height of about 100 nm.

The layered configuration of the center waveguide 140 forms a MOScapacitor comprising the layers 145, 146, 147, and 148, which allows anelectronic accumulation region to form at the interface between thetransparent conducting oxide layer 146 and the semiconductor oxide layer147 upon the application of a voltage bias between the metal layer 148and the silicon layer 145 via the respective electrodes 162, 164.

In an exemplary alternative embodiment of the waveguide 140, theelectrode 162 is coupled to the transparent conducting oxide layer 146rather than to the silicon layer 145 to avoid additional optical lossesof highly-doped semiconductors. In this configuration, when a voltagebias is provided between the metal layer 148 and the transparentconducting oxide layer 146, the electronic accumulation layer is stillformed at the interface between the transparent conducting oxide layer146 and the oxide layer 147. In this configuration, the MOS capacitorcomprises the layers 146, 147, and 148. In another embodiment of theinvention, the electrode 162 can be coupled to the ITO layer. However,it is desirable to have ITO in the optical low-loss state for no appliedvoltage. This state has the downside of being electrically of highresistance, which increases the contact resistance, which leads to highvoltages and low modulation speeds. However, a selective Oxygen plasmatreatment of ITO can make the ITO being low-resistive at any selectedcontact regions.

In accordance with an exemplary embodiment of the waveguide 140, thesemiconductor layer 145 is formed from silicon; the transparentconducting oxide layer 146 is formed from ITO; the oxide layer 147 isformed from SiO₂; and the metal layer 148 is formed from aluminum. Uponapplying a voltage bias between the aluminum layer 148 and the siliconlayer 145 via the respective electrodes 162, 164, the electronicaccumulation layer forms at the ITO 146-SiO₂ 147 interface. In anexemplary variation on this embodiment, the electrode 162 is coupled tothe ITO layer 146. In this configuration, when a voltage bias isprovided between the metal layer 148 and the ITO layer 146, theelectronic accumulation layer is still formed at the interface betweenthe ITO layer 146 and the SiO₂ layer 147. Alternatively, the electrodes162, 164 can be placed at the TCO layer (146) and the metal layer (148).The relative placement position of the electrodes 162, 164 along thex-axis is not critical.

In each of the exemplary embodiments of the waveguide 140 discussedabove, the MOS capacitor formed in the waveguide 140 offers a strongoptical confinement, which is significantly below the diffraction limitsof light, and strong electrostatics, such that a low bias voltage isneeded to switch the device 100. When the waveguide 140 is unbiased, theeffective index of the transparent conducting oxide layer 146 causes itto be a dielectric, in accordance with the following:n _(ITO-CROSS)=1.92−0.001i.  (1.)

When a voltage bias is applied to the waveguide 140 via the electrodes162, 164, the effective index of the transparent conducting oxide layer146 achieves a “quasi” metal state, in accordance with the following:n _(ITO-BAR)=1.042−0.273i.  (2.)

The former case (unbiased) defines the physical switch length, L, of thecenter waveguide 140 (FIG. 2B), which is the desirable coupling length.

The center waveguide 140 selectively controls the direction of lightthat enters the end waveguides 120, 130. For instance as illustrated inFIG. 2C, light that enters the input end 121 of the first waveguide cantravel in first direction shown by dashed line 150 along the entirelength of the first waveguide 120 to the output end 122. And, lightentering the input end 131 of the second waveguide 130 can travel alongthe entire length of the second waveguide 130 to the output end 132. Thewaveguides 120, 130 are not coupled, i.e., they are decoupled, whenlight enters the input port 121 and exits the waveguide 120 at theoutput port 122, or when light enters the input port 131 and exits thewaveguide 130 at the output port 132. When the waveguides 120, 130 arenot coupled, the device 100 is deemed to be in a bar state.

Alternatively, the light entering the input end 121 can travel in seconddirection 152. Here, the waveguides 120, 130 are coupled when lightenters the input port 121 of the first end waveguide 120, crosses overto the second end waveguide 130 through the center waveguide 140, andexits the second end waveguide 130 at the output port 132. Or, lightenters the input port 131 of the second end waveguide 130 and crossesover to the first end waveguide 120 through the enter waveguide 140, andexits the first end waveguide 120 at the output port 122. When thewaveguides 120, 130 are coupled, the device 100 is deemed to be in across state.

Coupling is achieved when the device 100 is unbiased. Thus, FIG. 2Cillustrates a path 152 that the light takes between the input port 121and the output port 132 when the waveguides 120, 130 are coupled whenthe device is unbiased. FIG. 2C also illustrates a path 150 that thelight takes between the input port 121 and the output port 122 when thewaveguides 120, 130 are decoupled when the device is biased.

An eigenmode analysis approach is adopted at plane A′ in FIG. 1 and FIG.2A to map-out and optimize the coupling performance of the device 100.In the eigenmode simulation, there are five supermodes; two are TEpolarized light that are ignored here as both effective indices do onlymarginally change with a voltage bias, and three TM polarized supermodes(two symmetric modes, TM₁ and TM₂, and an anti-symmetric mode TM₃) areof interest to this analysis.

Thus, by controlling the voltages applied to the electrodes 162, 164,the center waveguide 140 controls whether the light entering the input121 travels along the first path 150 and exits the output 122 or crossesover the center waveguide 140 and exits the output 132. When a voltageis applied to the electrodes 162, 164, the device 100 is biased becausethe loss coefficient of the ITO layer 146 increases, which makes thecenter waveguide 140 a quasi “reflector”, and the light travels alongthe first path 150 and exits the output 122. When there is no voltage,the device 100 is unbiased, the center waveguide 140 is considered as adielectric, and the light travels along the second path 152 and exitsthe output 132.

The function description of each individual layer of the island regionis as follows, and can be divided into an electrical and an opticalanalysis. Electrically, layers 145, 147, and 148 form an MOS capacitor,whereas layer 147 is the electron flow prohibiting layer. Here layer 146is the active material layer, whose carrier concentration is changed dueto an accumulation of electrons. Optically, the island forms a hybridplasmon polariton waveguide mode. The benefits of this mode for theswitch are that the light is squeezed into a sub diffraction-limitedarea, which in turn enhances the light matter interaction of the system.This results in a strong optical index modulation of 146, which alsoalters the local island mode and the global supermode. The latterdetermines the overall switch performance (i.e. bar and cross states).

Referring now to FIG. 6, there is illustrated a method 600 of optimizingthe coupling performance of the device 100, in accordance with anexemplary embodiment of the present invention. The optimization methodin analyzing the device 100 is performed follows:

(i) Find the optimized coupling length, L, of the center waveguide 140for the cross state (without bias voltage) using the numerical 2Deigenmode results, which define the height of the semiconductor oxidelayer 147, Step 610. The Step 610 comprises two sub-steps 612 and 614.In order to get the most coupling efficiency, the height of the oxidelayer 147 is determined in the Step 612 by matching the effective TM₃mode index of the waveguide 140 with ½ times the difference between theTM₁ and TM₂ mode indices as:½(n _(TM1) +n _(TM2))=n _(TM3)  (3.)

Under this condition, L_(c) is calculated in the Step 614 based on thebias-changed effective mode index, Δn_(eff), between two TM waveguidemodes, TM₁ and TM₃, inside the waveguide 140 of the device 100, and isgiven by:

$\begin{matrix}{{L_{c} = \frac{\lambda}{2\Delta\; n_{eff}}},} & (4.)\end{matrix}$

where λ is the operating wavelength. In one embodiment, the length ofthe center waveguide 140 is shorter or equal to that of the endwaveguides 120, 130, though any suitable length can be provided. Oneobjective of the device is to operate at (a) a small on-chip footprint,and (b) with low electrical power consumption. Both are affected by thephysical device length. For (b), the device length relates to anelectrical capacitance, which in turn relates to the average energyconsumption of the device with Energy/bit=½ CV², where C is theelectrical capacitance and V the applied voltage. C, however, relates tothe capacitance formed by 148 and 146 (or 145), and is proportional tothe area spanned by the area in the xy-plane by the layers connected tothe electrodes. In summary, the shortness of the device length, i.e. thelength of the island 140, relates directly to the usefulness of thedevice.

(ii) Analyze the effect of changing the widths, g₁ and g₂, of the gaps125, 135, Step 620. Reducing the size of gap 125, 135 may produceundesired coupling from the end waveguides 120, 130 directly withoutgoing through the center waveguide 140. Conversely, increasing of thesize of the gaps 125, 135 leads to weaker coupling. Hence the requiredcoupling length, L, increases monotonically, as shown in FIG. 3A.Furthermore, the coupling length, L, of the bar state increases fasterthan that for the cross state, which introduces the basis of theswitching behavior of the device 100. An explanation for this phenomenonis two-fold. First, the refractive index of the transparent conductingoxide layer 146 changes due to the bias applied to the island 140altering the propagation constant of the supermode, preferably insidethe island 140. Second, the optical loss of the island 140 issignificantly increased. Thus, the island 140 part acts as a metal-likereflector, keeping the incoming light in the waveguide 120.

(iii) Analyze the effect of the width, w₃, of the waveguide 140 (FIG.3B) and the height, h₃, of the silicon layer 145 (FIG. 3C), Step 630.Note, the height, h₃, and width, w₃, of the silicon layer 145 relativeto that of the waveguides 120 or 130 is a “lever” in the device 100, andthis difference of the geometrical height, h₃, and width, w₃, isreferred to as “detuning.” The height, h₃, of the silicon layer 145 neednot be the same as the height, h₁, of the waveguide 120 or the height,h₂, of the waveguide 130. Detuning the height, h₃, of the silicon layer145 would give an even higher coupling length, L, ratio. This indicatesthat if the height, h₃, of the silicon layer 145 would be lowered belowthe cut-off wavelength of the waveguide 140, more of the electric fieldwould sit in the plasmonic section, which lower the effective index ofthe cross state, to produce the larger effective index change. Theoptical MOS mode has a tight optical confinement overlapping with theTCO (number) and Oxide layer 148. Modal overlap is preferred to lightpassing through without applied bias between the electrodes 162, 164. Inone embodiment the TCO layer 147 absorbs or reflects the light back toend waveguide 120 with bias between the electrodes 162, 164.

(iv) Calculate the resulting extinction ratio between the output ports122 and 132 for two states and insertion loss for each state based onthe following equations, Step 640:

$\begin{matrix}{{ER}_{CROSS} = {10{\log\left\lbrack \frac{{Power} - {{out}\mspace{14mu}({BAR})}}{{Power} - {{out}\mspace{14mu}({CROSS})}} \right\rbrack}}} & (5.) \\{{ER}_{BAR} = {10{\log\left\lbrack \frac{{Power} - {{out}\mspace{14mu}({CROSS})}}{{Power} - {{out}\mspace{14mu}({BAR})}} \right\rbrack}}} & (6.) \\{{IL}_{CROSS} = {10{\log\left\lbrack \frac{{Power} - {{out}\mspace{14mu}({CROSS})}}{{Power} - {in}} \right\rbrack}}} & (7.) \\{{IL}_{BAR} = {10{\log\left\lbrack \frac{{Power} - {{out}\mspace{14mu}({BAR})}}{{Power} - {in}} \right\rbrack}}} & (8.)\end{matrix}$

The thickness, h₄, of the transparent conducting oxide layer 146 alsoplays an important role in the optimization of the device 100. FIG. 4Ashows that stable coupling at the cross state can be produced with anincreased thickness, h₄, of the transparent conducting oxide layer 146,while significantly improving the bar state signal discriminationbetween the output ports 122, 132 of the waveguides 120, 130,respectively, at the same time as shown in FIG. 4B.

To verify the switch 100, the electric field profile distribution overthe switch 100 in the xy plane (illustrated in FIG. 1) is illustrated byusing 3D finite-difference time-domain simulation, as illustrated inFIGS. 5A and 5B. FIG. 5A shows a cross state without a voltage bias.Accordingly, the light is shown crossing from the first waveguide 120,through the center waveguide 140, to the second waveguide 130, andexiting the second waveguide 130. Referring jointly to FIG. 2C and FIG.5, the light exits out of the longitudinal side 124 (toward the frontend 121 in the embodiment shown) that faces the center waveguide 140.The center waveguide 140 is positioned with respect to first waveguide120 so that the light enters the first longitudinal side 143 of thecenter waveguide 140 (toward the front end 141 in the embodiment shown),and exits through the second longitudinal side 144 of the centerwaveguide 140 (toward the rear end 142 in the embodiment shown). Thelight then passes into the second end waveguide 130 through the firstlongitudinal side 133 (toward the output end 132 in the embodimentshown). Of course, any suitable configuration can be provided, and thelight can exit and enter along any part of the length of the waveguides120, 130, 140. In one embodiment of the invention, the light signalpasses through the middle layers 146 and/or 147.

FIG. 5B shows a bar state with a voltage bias. Here, the light travelsonly in the first waveguide 120 and does not cross over the centerwaveguide 140 to the second waveguide 130. Some light passes from thefirst waveguide 120 to the center waveguide 140, but the light does notpass over into the second waveguide 130.

Wavelength-division-multiplexing (WDM), one data and signal routingscheme in optical communications, has been established as a desiredmeans of delivering high-data bandwidths. In order to prevent devicefailure caused by an individual resonator-based design, a spectrumanalysis by scanning the wavelengths from 1.30 to 1.85 micrometer isapplied for testing broadband operation performance. The optimizedextinction ratio of 21 dB at the cross state and 7.3 dB at the bar stateis observed at 1.55 μm wavelength, respectively. It gives a promising Lratio of 35 and above for a 400 nm bandwidth. Thus, the device 100offers to comply with the future requirement for the applications of WDMarchitectures.

Important performance figures of the electro-optic (EO) switch 100 areoperating efficiency (E/hit) and bandwidth (i.e. speed). Both may beoptimized by varying different geometric parameters of the device 100.Compared with a conventional Mach-Zehnder or ring structure-basedoptical switch, the switch 100 is more compact at a length range inbetween 4.8 to 5.5 μm, which is about 100 times more compact thancurrent devices, and the insertion loss can be as low as 1.52 dB.Furthermore, the speed performance is estimated up to THz switching bycalculating the RC delay time for the waveguide 140 with a resistiveload of 50Ω to 500Ω. The energy per bit is low as 8.98 fJ. Thus, thedevice 100 has high potential application for ultra-compact photonicintegrated circuits and data routing. In an exemplary embodiment, theswitch 100 is connected and integrated seamlessly to a low-costdata-routing silicon-on-insulator platform.

An exemplary quantitative performance analysis of the device 100 isprovided in Table 1 below. Here, the device is operating at thewavelength of 1.55 μm. The gate oxide thickness varies from 5 to 25 nm.The bandwidth (BW) is calculated from BW=1/RC where R has values from 50to 500 Ohm. Energy per bit (E/bit) is calculated by E/bit=½ CV2, whereapplied voltage is 1 to 2 V and 2 to 3 V for graphene and ITO,respectively.

TABLE 1 QUANTITATIVE PERFORMANCE ANALYSIS E/bit L IL ER BW Energy DeviceLength - Insertion Extinction Bandwidth - per μm Loss - dB Ratio - dBTHz bit -fJ 5.03 CROSS 1.52 CROSS 21 10.0 (50 Ω) 8.98 BAR 2.94 BAR 7.31 1.0 (500 Ω)

The invention is described as controlling a light signal passing fromone of the inputs 121, 131 to one of the outputs 122, 132. It should berecognized however, that the center waveguide 140 can also control lighttraveling in reverse from the output side 122, 132 (which is now aninput side) to one of the input sides 121, 131 (which is now an outputside), such as along paths 150, 152. In addition, light can travel alongthe paths 150, 152 at the same time or at different times. Thus, forinstance, light can travel from input 121 to output 122 at the same timelight travels from input 131 to output 122. Or, light can travel fromoutput 122 to input 131 at the same time light travels from input 121 tooutput 132. In addition, while only a single center waveguide 140 isshown, there can be multiple center waveguides. And, while the inventionis shown and described as being a 2×2 switch, other size switches arewithin the spirit and scope of the invention, such as a cascaded 3×3switch or a 2×3 switch. And, the switch 100 can be utilized forsymmetric and anti-symmetric waveguide coupling.

As shown and described, the center waveguide 140 as the control node ofthe switch is constructed by four layers 145, 146, 147, 148, which areprovided in the designated order and arrangement. However, othersuitable number of layers can be provided and for instance additionallayers can be added to the center waveguide 140. For instance, one moreoxide layer can be added in between the bottom layer 145 and the TCOlayer 146 as passivation.

The performance of the switch is quantified by three parameters;physical footprint, modulation speed capability (i.e. device delay), andelectrical power consumed (i.e. energy/bit). In all metrics the device100 are an improvement. As mentioned above, in one embodiment of theinvention, the two end waveguides 120 and 130 have the height h₁ and h₂equals to 250 nm and width w₁ and w₂ equal to 450 nm. Layers 145, 146,147, 148 in the center waveguide 140 share a width w₃ equal to 300 nm.The bottom layer 145 in the center waveguide 140 obtains a height lessthan 250 nm, and 160 nm is preferred. The thickness of the ITO/TCO layer146 is in the range of 10 nm-80 nm, and 80 nm is preferred. The oxidelayer 147 varies from 5-25 nm, with 5 nm being preferred. The metallayer 148 is preferred to be 100 nm. For all of these preferred values,the switch 100 with a length of 5.03 μm produces an insertion loss of1.52 dB and extinction ratio of 21 dB when light going through path 152,and an insertion loss of 2.94 dB and extinction ratio of 7.31 dB whenlight going through path 150 at a speed of 1.0 THz to 10.0 THz. Theenergy per bit is 8.98 fJ. The insertion loss is improved, which theisland 140 bears higher optical losses per length compared tostate-of-the-art devices, which are typically pure photonic vs.plasmonic here), the actual physical length of the device 100 issignificantly shorter, thus the absolute loss is reduced by factorsranging between about 2-10. In another embodiment of the invention, theisland 140 layers 145-148 can be flipped around in the z-direction, i.e.the island 140 starts from the bottom with a metal layer and ends at thetop with a high dielectric layer.

It is noted that the switch 100 is especially useful in computing andnetworking hardware, such as network-on-chip designs (i.e. inter-core),or in optical communication links within a processing element (i.e.intra-core). However, other suitable applications can be made, withinthe spirit and scope of the invention.

The following documents are cited herein, and are hereby incorporated byreference: [1] R. Kirchain, and L. Kimerling Nature Photonics 1, 303-305(2007). [2] M. R. Watts, H. A. Haus, and E. P. Ippen Opt. Lett. 30(9),967-969 (2005). [3] D. Dai, J. Bauters, and J. E. Bowers Light: Sci.Appl. 1(3), 1-14 (2012). [4] F. Verhagen, M. Spasenovic, A. Polman, L.Kuipers, Phys. Rev. Lett. 102, 203904 (2009). [5] J. A. Dionne, H. J.Lezec, H. A. Atwater, Nano Lett. 6 (9), 1928 (2006). [6] V. J. Sorger,Z. Ye, R. F. Oulton, G. Bartal, Y. Wang, X. Zhang, Nat. Commun. 2, 331(2011). [7] S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. Laluet, T. W.Ebbesen, Nature 440, 508 (2006). [8] K. Y. Jung, F. L. Teixeira. R. M.Reano, IEEE Photonics Technol. Lett. 21 (10), 630 (2009). [9] D. F. P.Pile, T. Ogawa, D. K. Gramotnev, T. Okamoto, M. Haraguchi, M. Fukui, S.Matsuo, App. Phys. Lett. 87, 061106 (2005). [10] R. F. Oulton, G. Banal,D. F. P. Pile, X. Zhang, New J. Phys. (Plasmonics Focus Issue) 10,105018 (2008). [11] B. Steinberger, A. Hohenau, H. Ditlbacher, A. L.Stepanov, A. Drezet, F. R. Aussenegg, A. Leitner, J. R. Krenn, App. PhysLett. 88, 094104 (2006). [12] A. V. Krasavin, A. V. Zayats, Opt. Express18 (11), 11791 (2010). 24. R. M. Briggs, J. Grandidier, S. P. Burgos, E.Feigenbaum, H. A. Atwater, Nano Lett. 10 (12), 4851 (2010). [13] V. J.Sorger, R. F. Oulton, J. Yao, Banal and X. Zhang Nano Letters 9,3489-3493 (2009). [14] R. F. Oulton, V. J. Sorger, D. F. B. Pile, D.Genov, and X. Zhang Nature Photonics 2, 496-500 (2008). [15] V. J.Sorger, Z. Ye, R. F. Oulton, G. Bartal, Y. Wang, and X. Zhang NatureComm. 2, 331 (2011). [16] V. J. Sorger, D. Kimura, R. M. Ma and X. ZhangNanophotonics 1, 1, 17-22 (2012). [17] J. P. Donnelly, H. A. Haus, andN. Whitaker, IEEE J. Quantum Electron. 23, 401 (1987). [18] F. Lou, D.Dai, and L. Wosinski Opt. Lett. 37(16), 3372-3374 (2012). [19] JingyeeChee, Shiyang Zhu, and G. Q. Lo, Opt. Express 20, No. 23, 5 (2012). [20]E. D. Palik, “Handbook of Optical Constants of Solids,” Academic Press.

These and other advantages of the present invention will be apparent tothose skilled in the art from the foregoing specification. Accordingly,it is to be recognized by those skilled in the art that changes ormodifications may be made to the above-described embodiments withoutdeparting from the broad inventive concepts of the invention. It is tobe understood that this invention is not limited to the particularembodiments described herein, but is intended to include all changes andmodifications that are within the scope and spirit of the invention.

What is claimed is:
 1. An electro-optical switch comprising: a lowdielectric layer serving as a substrate; a first high dielectricwaveguide disposed on the low dielectric layer; a second waveguidedisposed on the low dielectric layer; and a third waveguide disposed onthe low dielectric layer, the third waveguide comprising: a transparentconducting oxide layer; a low dielectric layer; a metal layer; and apair of electrodes coupled to the third waveguide and configured to biasthe third waveguide to effect optical switching in the electro-opticalswitch.
 2. The electro-optical switch of claim 1, wherein the lowdielectric layer is disposed between the transparent conducting oxidelayer and the metal layer.
 3. The electro-optical switch of claim 1,wherein the transparent conducting oxide layer is formed fromindium-tin-oxide and the metal layer is formed from aluminum or anylow-resistive metal.
 4. The electro-optical switch of claim 1, whereinthe first waveguide is separated from the third waveguide by a first gaphaving a first width, and the second waveguide is separated from thethird waveguide by a second gap having a second width.
 5. Theelectro-optical switch of claim 4, wherein the first and second widthsare equal.
 6. The electro-optical switch of claim 1, wherein the firstwaveguide comprising an input for receiving light and an output foroutputting light when the electro-optical switch is in a cross state,and wherein the second waveguide comprises an output for outputting thelight received at the input when the electro-optical switch is in a barstate.
 7. The electro-optical switch of claim 6, wherein for a lightsignal input having a wavelength from 1.30 to 1.85 micrometer, a ratioof coupling length of the electro-optical switch in the bar state to acoupling length of the electro-optical switch in the cross state is 35and greater for a 400 nm bandwidth.
 8. The electro-optical switch ofclaim 6, wherein an extinction ratio between the output of the firstwaveguide and the output of the second waveguide in the cross state is21 dB and 7.3 dB in the bar state when the light has a 1.55 μmwavelength.
 9. The electro-optical switch of claim 6, wherein aninsertion loss in the cross state is about 1.5 dB.
 10. Theelectro-optical switch of claim 6, wherein an energy per bit is about9.0 fJ.
 11. The electro-optical switch of claim 1, wherein the thirdwaveguide further comprises a semiconductor layer disposed between thesubstrate and the transparent conducting oxide layer.
 12. A method forcoupling a light signal between the first, second, and third waveguides,the method comprising steps of: providing a first waveguide configuredto carry an optical signal; providing a second waveguide configured toreceive the optical signal from the first waveguide; providing a thirdwaveguide positioned at least partially between the first waveguide andthe second waveguide, wherein the third waveguide has a transparentconducting oxide layer, low dielectric layer, and metal layer; andselectively transferring by the third waveguide, the optical signal fromsaid first waveguide to said second waveguide.
 13. The method of claim12, wherein the step of selectively comprises coupling a pair ofelectrodes to the third waveguide and controlling the pair of electrodesto bias the third waveguide to effect optical switching.
 14. The methodof claim 12, further comprising optimizing coupling between the first,second and third waveguides by: performing an eigenmode analysis at across-section through the first, second, and third waveguides;determining an optimized coupling length for a cross state of theelectro-optical switch; analyzing an effect of changing a width of afirst gap between the first and third waveguides and a width of a secondgap between the second and third waveguides to select an optimal valuefor the width of the first gap and an optimal value for the width of thesecond gap; analyzing an effect of changing a width of the thirdwaveguide and a height of the low dielectric layer to select an optimalvalue for the width of the third waveguide and an optimal value for theheight of the low dielectric layer; and calculating an extinction ratiobetween an output port of the first waveguide and an output port of thesecond waveguide for the electro-optical switch having optimal valuesfor the width of the first gap, the width of the second gap, the widthof the third waveguide, and the height of the low dielectric layer. 15.The method of claim 14, wherein the step of determining comprises a stepof calculating a height of the low dielectric layer of the thirdwaveguide.
 16. The method of claim 14, wherein the step of determiningcomprises steps of: calculating a height of the low dielectric layer ofthe third waveguide by matching an index of an anti-symmetricTM-polarized mode with one half of a difference between indices of firstand second symmetric TM-polarized modes; and calculating the optimizedcoupling length for the cross state of the electro-optical switch basedon a bias-changed effective mode index between the first and secondsymmetric TM-polarized modes inside the third waveguide based on anoperating wavelength.
 17. The method of claim 14, further comprising astep of analyzing an effect on coupling by varying a thickness of thetransparent conducting oxide layer.
 18. A light routing switchcomprising: a first waveguide configured to carry an optical signal; asecond waveguide configured to receive the optical signal from saidfirst waveguide; and, a third waveguide positioned at least partiallybetween said first waveguide and said second waveguide, said thirdwaveguide selectively transferring the optical signal from said firstwaveguide to said second waveguide, wherein said third waveguidecomprises a first layer of metal, a second layer of an oxide, a thirdlayer of a transparent conductive oxide, and a fourth layer of a highdielectric, with an alternative design option that is similar theaforementioned one, but is comprised with an added second oxide betweenthe transparent conductive oxide, and the high-dielectric underneath.19. The router of claim 18, wherein said first, second and thirdwaveguides are elongated and are arranged substantially parallel to eachother.
 20. An electro-optical switch comprising: a low dielectric layerserving as a substrate; a first high dielectric waveguide disposed onthe low dielectric layer; a second waveguide disposed on the lowdielectric layer; and a third waveguide disposed on the first lowdielectric layer, the third waveguide comprising: a transparentconducting oxide layer; a low dielectric layer; a metal layer; andswitching means controlling the effective modal index of the thirdwaveguide via an electrical bias to the metal and TCO or alternatively,to the metal and bottom low-dielectric in the third waveguide to effectoptical routing.